It is often necessary to noninvasively determine various physical characteristics of the fluid contents inside sealed containers such as drums, reactor vessels, railroad tanker cars, holding tanks, and the like. Typically the information required consists of fill level and liquid density. As an example, there are hundreds of thousands of 55-gallon drums at various locations in the United States which are filled with hazardous waste materials, and other bulk containers contain chemical (weapons) nerve agents. In many cases, records as to the nature of the contents have been lost or destroyed. These drums are often stacked on top of one another in several rows and can be accessed only from the side of the container for testing. Prior to disposal, it is important to obtain an idea as to the liquid level and density. Another example includes the maintenance of current inventories of chemical-filled railroad tanker cars in order for the chemical industry to control customer shipments. Additionally, there is a need for liquid level monitoring in reactor vessels for accurate process control.
There are numerous liquid level measuring devices currently available. However, most of these devices require invasion of the containing vessel by placing sensors therein or by arranging for windows through which observations may be made. For example, in R. M. Langdon, "Vibratory Process Control Transducers," Electronic Engineering, pages 159-168 (November 1981), the authors disclose that flexural vibration of metal structures inserted into liquids exhibits vibrational frequencies dependent upon liquid level. A rigid metal rod or tube of suitable cross section is inserted into the liquid with a piezoelectric or electromagnetic transducer attached to it to generate flexural vibrations at frequencies in the kilohertz range. The speed with which the vibrations pass along the rod is constant for a given frequency and depends on such things as the density and elastic constants of the metal and the rod dimensions. It also depends on the nature of the material surrounding the rod so that when the rod is immersed in a liquid, a significant reduction in rod-wave velocity occurs. Consequently, when the rod is mounted in such a way that it can resonate in a flexural mode, the transit time of waves passing along the rod and hence the frequency of vibration is reduced by immersion in a liquid. The change in resonant frequency is approximately linearly dependent on the fraction of the rod immersed so this frequency provides a measure of liquid level. A similar device is described in Hyok S. Lew, "Resonance Frequency Liquid Level Sensor," U.S. Pat. No. 5,074,148, which issued on Dec. 24, 1991.
Another widely used ultrasonic technique for fill-level monitoring in containers is echo-ranging. The most common adaptation of this technique employs an air transducer to direct a beam of ultrasound through the air above the surface of the liquid downward to the liquid surface. The reflected ultrasonic signal is detected either by the same transducer or by a second transducer, the propagation time through the air space providing a direct and continuous measure of the fill level in the container. This invasive approach cannot be used for sealed containers unless the transducers in the air space directly above the liquid surface are placed there before the liquid is introduced to the container, since an ultrasonic wave cannot be adequately coupled into the air in the container from outside thereof. Unless complex reference subtraction techniques are employed, the accuracy of the technique suffers from the fact that the sound velocity in the medium above the liquid whose level is to be determined varies with temperature and composition of the gas therein.
If the liquid has a fixed composition, the pulse-echo sensor may be mounted at the bottom of the container and outside of it. A signal may be generated in the liquid through the container wall for continuous fill-level monitoring using the return propagation time through the liquid to the liquid/vapor interface. A difficulty with this method arises if the bottom of the drum cannot be readily accessed. Moreover, sound speed is affected by liquid temperature, and highly attenuating liquids or long pathlengths may completely absorb the signal. Additionally, the presence of bubbles or other suspended particles will prevent the signal from entering the bulk of the liquid because of sound scattering. Contamination of the liquid by impurities, liquid stratification, and poor coupling of transducers to the container surface will all adversely affect such measurements.
A variation of this noninvasive ultrasonic technique introduces pulsed ultrasonic waves into a vessel at various levels through the side of the container and observes the echo from the opposite side of the container. When the liquid level is exceeded, there is no return signal or echo. The ultrasonic pulse is introduced into the container using a transducer attached to the surface thereof using a coupling gel or some other coupling means. Below the liquid level, the pulse propagates through the liquid, bounces off of the other side of the container and is detected by the same transducer. Again, the return signal may be severely attenuated in the situation where the liquid is strongly absorbing at the frequency employed or has substantial numbers of suspended particles which are effective in scattering sound. A similar effect occurs when the container is very large.
Other noninvasive techniques include nuclear or x-ray imaging which are expensive, nonportable, and complex.
For sealed containers, such as chemical storage drums, then, there does not appear to be a reliable way to determine liquid level and density of the contents without opening them.
Accordingly, it is an object of the present invention to provide an accurate, noninvasive method for measuring the fill level of liquids in containers.
Another object of the invention is to provide an accurate, noninvasive method for measuring the fill level of liquids in containers independent of the size and shape thereof.
Yet another object of the present invention is to provide an accurate, noninvasive method for measuring the density of liquids in containers.
Still another object of the invention is to provide an accurate, noninvasive method for measuring the density of liquids in containers independent of the size and shape thereof.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.